
Then we have (cf.[1]}), in the coordinate frame \((\xi _j,t)\) ,
\(u^{}_{\mathrm {2SS}}\sim u^{}_{\mathrm {1SS}}[\xi _{j}+\Delta \xi _{j}^{(\pm )};\mathcal {X}_{j}+\Delta \mathcal {X}_{j}^{(\pm )}],~~ t \rightarrow \pm \infty ,~~ (j=1,2)\) 
